1. Field of the Invention
The invention relates to an electronic circuit with a voltage and/or current reference circuit.
2. Description of Related Art
Such a circuit is known from an article titled "New class of high-performance PTAT current sources", by H. C.Nauta and E. H.Nordholt, published in Electronics letters Vol. 21 No. 9 pages 384 to 386, April 1985 (the Nauta article). FIG. 1 shows a PTAT reference circuit disclosed in the Nauta article.
At the core of this PTAT reference circuit are two transistors and a resistor. Furthermore, the circuit disclosed in the Nauta article uses two (high impedance) current sources. The current sources on the one hand and the transistors and the resistor on the other hand are connected to opposite power supply poles. Thus the current sources are able to supply proportionally adjustable currents I to the transistors and the resistor (that is, the currents are adjusted so that the proportion between these currents remains fixed).
The PTAT reference circuit makes use of the logarithmic relation between base emitter voltage Vbe and junction current density i of bipolar transistors: EQU Vbe=kT/q log i/i0
Here "log" is the natural logarithm and i0 is a standard current density which is substantially the same for any transistor. In the known PTAT reference circuit unequal current densities i1, i2 (where i1=n*i2) are supplied to two transistors by supplying the same current I to two transistors whose junction area differs by a factor n. As a result, there is a fixed difference dV between the base emitter voltages in the two transistors: EQU dV=kT/q log n
At the same time, the current I is fed through a resistor R, so that a voltage drop IR occurs through the resistor. A feedback loop adjusts the current supplied by the current sources so that the voltage drop compensates the dV difference between the junction. i.e. so that EQU IR=kT/q log n
Thus a reference current I is obtained.
The circuit disclosed in the Nauta article uses two (high impedance) current sources to supply the current I to the two transistors. This is in contrast to more conventional reference circuit designs, which use the (low impedance) input and (high impedance) output of a current mirror to supply the current I to respective ones of the transistors. By the use of two high impedance current sources, the Nauta article achieves high accuracy because it overcomes the detrimental consequences (e.g. supply voltage dependence) of the Early effect on the accuracy of the reference circuit.
However, it has been found that the reference circuit disclosed in the Nauta article has a potential instability problem, which can be overcome only by cumbersome additional circuits such as adding a relatively large capacitor between point A and Vnn. This capacitor undoes the elimination of the detrimental consequences of the Early effect at higher frequencies, because it causes an imbalance between the loads of the current sources; moreover the capacitor takes up circuit space.